In pre-SDN architectures, although there is a separation between the control plane and the data plane, the communication between the layers is usually proprietary and it is defined by the merchant silicon vendor or network equipment vendors. Moreover, the network management configuration syntax is not interoperable between networking gear from different vendors, for instance, CLI for Cisco IOS is not compatible with Juniper's JUNOS CLI.
The definition of standard protocols with open APIs between control plane and data plane in addition to a standardized configuration model is the catalyst that is enabling the SDN. This standard API-based separation of the control plane from the forwarding plane enables consistent management and control across network. It also facilitates interoperability between forwarding planes and control planes between networking equipment from different vendors. The ability to communicate with forwarding elements across the network...